Two of the salient features of the fetal alcohol syndrome in humans and rat models are microencephaly and mental retardation/learning deficits. Rats prenatally exposed to ethanol (such that mean peak blood ethanol concentrations reach 150-180 mg/dl), have brains which are significantly (11%) smaller than the offspring of pair-fed controls. Moreover, the somatosensory cortex of ethanol-treated rats have fewer (33%) neurons than controls. In the first 3 years of this project, we have shown that the reduction in the size of the cerebral cortex resulting from chronic exposure to ethanol during the second half of gestation results from profound alterations in the proliferation of neural cells. Interestingly, cortex is derived from cells in 2 anatomically discrete proliferative zones and these zones are differentially affected by ethanol. During the next 5 years, we propose to build on this bank of data and test the hypothesis that a primary target of ethanol toxicity is proliferating neural cells. Three sets of experiments will be performed. (1) We will examine the effect of limiting ethanol exposure to times when a particular proliferating population is active. We will determine if there is a critical period for the deleterious effects of ethanol exposure on neocortical cell proliferation. Rats will be acutely exposed to ethanol during periods (for 5 days) when stem cells are proliferating and when each of the cortical proliferative zones is pre-eminent. In a corollary study, we will ascertain how generalizable these effects are by studying the effects of pre- or postnatal exposure to ethanol on the generation of neurons in the hippocampal formation and cerebellum (structures which are also derived from 2 proliferative zones). (2) The effects of ethanol on the fraction of cells that permanently leave the population of cycling cells (to commence migrating to cortex) will be ascertained. (3) The microenvironments of the 2 proliferative zones are largely established by the fluid-brain barriers. Therefore, we will examine (a) the effect of ethanol on these barriers as well as (b) the effect of varying blood ethanol concentration on proliferating populations in the neocortex, the hippocampal formation, and the cerebellum. Thus, this proposal describes studies which will assess the cellular and systemic mechanisms which underlie ethanol teratogenicity. With the data accrued in FY04-FY08, we expect to be able to predict the range and complexity of the ethanol-induced defects in the CNS based upon the timing and amount of the ethanol exposure.